ProtozoaAmoebae

Entamoeba histolytica

• Epidemiology: Worldwide; tropics, poor sanitation.
• Transmission: Ingestion of cysts (faeco-oral).
• Clinical: Amoebic dysentery; amoebic liver abscess ("anchovy sauce pus"); rare pulmonary/brain spread.
• Diagnosis:
  • Microscopy (trophozoites with ingested RBCs).
  • PCR/antigen (distinguishes from E. dispar).
  • Serology (useful in liver abscess).
  • Imaging (US/CT) helpful in detecting liver abscess.
• Treatment:
  • Metronidazole + luminal agent (paromomycin/diloxanide).
  • Drain abscess if risk of rupture or poor response to therapy;

Acanthamoeba spp.

• Epidemiology: Free-living amoeba in soil, water, contact lens solutions.
• Transmission: Traumatic inoculation into cornea; inhalation/cutaneous entry → CNS (immunocompromised).
• Clinical:
  • Keratitis: severe, painful, ring infiltrates in contact lens wearers; often misdiagnosed as HSV keratitis.
  • Granulomatous amoebic encephalitis (GAE): subacute/chronic in immunocompromised; weeks–months.
• Diagnosis: Corneal scrapings: culture on non-nutrient agar with E. coli overlay; PCR; confocal microscopy.
• Treatment:
  • Keratitis: prolonged topical therapy (biguanides, diamidines, ± azoles).
  • GAE: multi-drug regimens (miltefosine, pentamidine, azoles, flucytosine, macrolides). Poor prognosis.

Naegleria fowleri

• Epidemiology: Free-living; warm freshwater lakes/rivers, poorly chlorinated pools. Rare but fulminant.
• Transmission: Water forced into nasal passages → olfactory mucosa → CNS. Not by ingestion.
• Clinical: Primary amoebic meningoencephalitis (PAM): acute, rapidly progressive; death in days.
• Diagnosis: CSF: neutrophilic pleocytosis, ↓ glucose, ↑ protein; wet mount: motile trophozoites (distinguishes from Acanthamoeba); PCR confirmation. Culture possible on non-nutrient agar with bacteria, though rarely used.
• Treatment: Amphotericin B (IV + intrathecal), rifampicin, azoles, miltefosine; early aggressive therapy + hypothermia sometimes successful. Mortality >95%.
• Prevention: Avoid nasal exposure to warm freshwater; proper pool chlorination.
• 

Balamuthia mandrillaris

• Epidemiology: Rare; soil exposure.
• Transmission: Inhalation or skin → CNS.
• Clinical: Granulomatous amoebic encephalitis; cutaneous lesions.
• Diagnosis: Histology (amoebic trophozoites and cysts), PCR.
• Treatment: Multi-drug regimens (miltefosine, azoles, macrolides, flucytosine); prognosis poor.

Intestinal Protozoa

Giardia lamblia (intestinalis, duodenalis)

• Epidemiology: Worldwide; waterborne outbreaks, esp. travellers/children.
• Transmission: Ingestion of cysts from contaminated water/food.
• Clinical: Malabsorptive diarrhoea, bloating, steatorrhoea; chronic infection → growth failure in children.
• Diagnosis: Stool microscopy (trophozoite with “falling leaf motility”); antigen detection ELISA, PCR; string test (Entero-test) is a classical though rarely used method.
• Treatment: Metronidazole, tinidazole, or nitazoxanide.

Cryptosporidium (C. hominis, C. parvum)

• Epidemiology: Worldwide; zoonotic reservoirs. Oocysts chlorine-resistant → waterborne outbreaks.
• Transmission: Ingestion of oocysts (faeco-oral, contaminated water, animal contact). Oocysts are immediately infectious on excretion.
• Clinical:
  • Immunocompetent: watery diarrhoea, self-limited.
  • Immunocompromised: chronic diarrhoea, biliary/pancreatic involvement.
• Diagnosis: Modified Ziehl–Neelsen (acid-fast oocysts 4–6 μm); antigen assays; PCR.
• Treatment: Supportive; nitazoxanide (variable efficacy); optimise ART in HIV.
• Infection control: Strict hygiene, water filtration; chlorine insufficient.

Cyclospora cayetanensis

• Epidemiology: Tropics/subtropics; foodborne outbreaks (berries, salad).
• Transmission: Ingestion of oocysts (require sporulation outside host to become infectious).
• Clinical: Relapsing/prolonged watery diarrhoea, esp. in travellers or immunocompromised.
• Diagnosis: Variable acid-fast oocysts (8–10 µm, larger than Cryptosporidium); autofluorescence; PCR.
• Treatment: Co-trimoxazole.

Cystoisospora (Isospora) belli

• Epidemiology: Rare; immunocompromised (HIV).
• Transmission: Ingestion of oocysts (require sporulation outside host to become infectious).
• Clinical: Watery diarrhoea, malabsorption, weight loss.
• Diagnosis: Large acid-fast oocysts (20–30 µm, larger than Cryptosporidium and Cyclospora); PCR.
• Treatment: Co-trimoxazole.

Balantidium coli

• Epidemiology: Rare; zoonosis (pigs).
• Transmission: Ingestion of cysts (faeco-oral).
• Clinical: Dysentery-like illness; colitis.
• Diagnosis: Large ciliated trophozoites in stool.
• Treatment: Tetracycline; alternatives: metronidazole, iodoquinol.

Urogenital Protozoa

Trichomonas vaginalis

• Epidemiology: Worldwide; common STI.
• Transmission: Sexual contact.
• Clinical:
  • Women: vaginitis, frothy discharge, “strawberry cervix.”
  • Men: urethritis, prostatitis (often asymptomatic).
• Diagnosis: Wet mount (motile trophozoites, “jerky motility”); antigen/PCR.
• Treatment: Metronidazole or tinidazole (treat partners).

Blood & Tissue Protozoa

Plasmodium spp. (falciparum, vivax, ovale, malariae, knowlesi)

• Epidemiology: Endemic in tropics/subtropics.
• Transmission: Anopheles mosquito.
• Clinical: Malaria: cyclical fever, anaemia, splenomegaly; falciparum → severe malaria (cerebral, renal failure, acidosis). Vivax and ovale can relapse due to dormant hypnozoites in liver.
• Diagnosis: Thick/thin blood films (species ID); rapid diagnostic tests; PCR.
• Treatment:
  • Uncomplicated: artemisinin-based combination therapy (ACT).
  • Severe: IV artesunate.
  • Primaquine required to eradicate hypnozoites (vivax/ovale).
• Prevention: Chemoprophylaxis, vector control.

Babesia (B. microti, B. divergens)

• Epidemiology: Tick-borne (Ixodes). B. microti (US); B. divergens (Europe, severe in splenectomised).
• Transmission: Tick bite; transfusion; rarely congenital.
• Clinical: Malaria-like fever, haemolysis; severe in splenectomised, elderly, immunocompromised.
• Diagnosis: Blood film: intraerythrocytic rings, Maltese cross tetrads; lacks haemozoin pigment (unlike malaria). PCR; serology.
• Treatment:
  • Mild: atovaquone + azithromycin.
  • Severe: clindamycin + quinine; exchange transfusion if high parasitaemia.
• Prevention: Tick avoidance; donor blood screening.

Trypanosoma brucei (gambiense, rhodesiense)

• Epidemiology: Sub-Saharan Africa; vector: tsetse fly (Glossina).
• Transmission: Bite of infected fly.
• Clinical: African trypanosomiasis (“sleeping sickness”):
  • Chancre at bite site.
  • Haemolymphatic stage: fever, lymphadenopathy (Winterbottom’s sign).
  • CNS stage: somnolence, confusion, coma.
• Diagnosis: Blood/CSF microscopy (trypomastigotes); serology (CATT test for T. b. gambiense).
• Treatment:
  • New: Fexinidazole is an oral nitroimidazole recently licensed for all stages of T. b. gambiense sleeping sickness.

	gambiense	rhodesiense
Early	pentamidine	suramin
CNS	eflornithine ± nifurtimox	melarsoprol

• Additional note: Pathogenesis involves antigenic variation of surface glycoproteins.
• Key differences:
  • T. b. gambiense: West/Central Africa, chronic disease (months–years), human reservoir, more common; CNS involvement late; better prognosis.
  • T. b. rhodesiense: East/Southern Africa, acute/rapid course (weeks), zoonotic reservoir (cattle, game), more severe, early CNS involvement; high mortality if untreated.

Trypanosoma cruzi

• Epidemiology: Latin America; vector: reduviid “kissing bug.”
• Transmission: Contamination of bite by vector faeces; transfusion; congenital.
• Clinical:
  • Acute: fever, chagoma, Romana’s sign (periorbital oedema).
  • Chronic: cardiomyopathy, arrhythmia, megaoesophagus, megacolon.
• Diagnosis: Acute: blood film (trypomastigotes). Chronic: serology, PCR. Xenodiagnosis is a traditional though rarely used method.
• Treatment: Benznidazole or nifurtimox (more effective in acute).

Leishmania spp.

• Epidemiology: Mediterranean, Middle East, Asia, South America; vector: sandfly.
• Transmission: Bite of infected sandfly.
• Clinical:
  • Visceral leishmaniasis (kala-azar): fever, weight loss, hepatosplenomegaly, pancytopenia.
  • Cutaneous leishmaniasis: localised skin ulcers at bite site.
  • Mucocutaneous leishmaniasis: destructive mucosal lesions (nasopharyngeal).
• Diagnosis: Amastigotes (Leishman-Donovan bodies) in macrophages; PCR; serology; rk39 antigen test used for visceral disease.
• Treatment:
  • Visceral leishmaniasis: liposomal amphotericin B (first-line); alternatives: miltefosine, pentavalent antimonials (resistance in India).
  • Cutaneous leishmaniasis: often self-limiting; local therapies (cryotherapy, intralesional antimonials); systemic treatment (miltefosine, amphotericin B) for severe, disseminated, or mucocutaneous disease.

Zoonotic / Opportunistic Protozoa

Toxoplasma gondii

• Epidemiology: Worldwide; cats are definitive hosts.
• Transmission: Ingestion of oocysts (cat faeces) or tissue cysts (undercooked meat); congenital.
• Clinical:
  • Immunocompetent: mild flu-like, lymphadenopathy.
  • HIV: encephalitis (multiple ring-enhancing brain lesions).
  • Congenital: triad (chorioretinitis, hydrocephalus, intracranial calcifications).
• Diagnosis: Serology (IgM/IgG, avidity); PCR (blood, CSF); imaging in HIV.
• Treatment: Pyrimethamine + sulfadiazine + folinic acid. Spiramycin in pregnancy to reduce congenital transmission.

Microsporidia (Enterocytozoon bieneusi, Encephalitozoon spp.)

• Epidemiology: Opportunistic, esp. HIV/immunosuppressed.
• Transmission: Ingestion or inhalation of spores.
• Clinical: Chronic diarrhoea, wasting; keratitis; disseminated disease.
• Diagnosis: Modified trichrome stain; EM; PCR. Spores are very small (1–2 μm).
• Treatment: Albendazole (variable species activity); ART in HIV.

Key Buzzwords for Exams

• Entamoeba: “anchovy sauce pus.”
• Giardia: “falling leaf motility.”
• Cryptosporidium: “acid-fast oocysts, swimming pools.”
• Cyclospora: “autofluorescent oocysts.”
• Cystoisospora: “large acid-fast oocysts.”
• Balantidium: “large ciliate trophozoite.”
• Trichomonas: “strawberry cervix.”
• Plasmodium falciparum: “banana-shaped gametocyte.”
• Babesia: “Maltese cross; no pigment.”
• T. brucei: “Winterbottom’s sign; antigenic variation.”
• T. cruzi: “Romana’s sign.”
• Leishmania: “amastigotes in macrophages.”
• Naegleria: “motile trophozoites in wet CSF.”
• Acanthamoeba: “ring infiltrate keratitis.”
• Toxoplasma: “ring-enhancing brain lesions in HIV; spiramycin in pregnancy.”
• Microsporidia: “tiny 1–2 μm spores.”